The present application relates to cryocatheters and wands, i.e. to catheters and wands which are used to locally ablate tissue by extreme cooling contact. Such implements, henceforth generically referred to herein as “cryocatheters” or simply “catheters” may, for example, have an elongated body through which a cooling fluid circulates to a tip portion which is adapted to contact and freeze tissue. The invention also relates to cold-mapping catheters. In general, such catheters may be used to lower the temperature of tissue, such as cardiac wall tissue, to an extent such that signal generation or conduction ceases and allows one to map or confirm that the catheter is positioned at a particular lesion or arrhythmia conduction site. Cryocatheters, however, operate at lower temperatures, and are configured for ablation treatment, to cool the tissue to a level at which freezing destroys the viability of the tissue, and, in the case of cardiac tissue, permanently removes it as a signal generating or signal conducting locus. Such devices are also useful for tissue destruction in other contexts, such as the ablation of tumorous, diseased, precancerous or congenitally abnormal tissue. The invention also relates to electrically driven catheters, such as RF ablation catheters. These catheters have an arrangement of one or more electrodes at their tip configured to contact tissue and apply RF energy thereto so that the tissue heats up due to resistive heating, creating an ablation lesion that may extend to a depth of several millimeters or more. Such catheters have sometimes been equipped with coolant supplies in the prior art to cool the tip and prevent electrode charring, or to cool adjacent tissue and perform cold-mapping and ablation with the same instrument.
Cryo and RF ablation catheters create lesions of different characteristics, and in a cardiac setting, one type may be preferred for treating lesions. Thus, freezing lesions may take longer to generate, allowing the operator to terminate the ablation to avoid adverse effects, and the lesions may be of lesser extent, so that they heal more quickly. These factors may dictate choosing a cryoablation catheter when the treatment sites are located in a thin cardiac wall. For open surgery the particular limitations or benefits of one or the other catheter may be addressed by special constructions, such as providing two-sided tissue contacting plates for cooling or for providing RF energy through the target tissue. However, for endovascular use each type of catheter remains subject to distinct limitations.
Cryocatheters may be adapted for endovascular insertion, or for insertion along relatively confined pathways, for example through a body lumen, or through a small incision to and around intervening organs, to reach an intended ablation site. As such, they are characterized by a relatively elongated body through which the cooling fluid must circulate, and a tip or distal end portion where the cooling is to be applied. The requirement that the coolant be localized in its activity poses stringent constraints on a working device. For example when the catheter contact must chill tissue to below freezing, the coolant itself must attain a substantially lower temperature. Furthermore the rate of cooling is limited by the ability to supply a sufficient mass flow of coolant and to circulate it through the active contact region, and the efficacy of the contact region itself is further limited by geometry and physical properties that affect its ability to conduct heat into the tissue. The rate of cooling may change depending upon the effectiveness of thermal contact, e.g. upon the contact area and contact pressure between the catheter and the tissue, and may be further influenced by ice accumulations or other artifacts or changes due to the freezing process itself. Moreover, it is a matter of some concern that proximal, adjacent or unintended tissue sites should not be exposed to harmful cryogenic conditions. These somewhat conflicting requirements make the actual implementation of an effective cryocatheter complex. One such device treats or achieves a relatively high rate of heat transfer by providing a phase change coolant which is pumped as a liquid to the tip of the catheter and undergoes its phase change in a small chamber located at the tip. The wall of the chamber contacts adjacent tissue directly to effect the cooling or ablation treatment. By employing a phase change refrigerant injected at ambient temperature along the body of the catheter to undergo expansion at the tip, the cooling effect may be restricted to the localized treatment region surrounding the tip portion of the device. The dimensions of catheter construction, particularly for an endovascular catheter, require that the phase change coolant be released from a nozzle or tube opening at a relatively high pressure, into a relatively small distal chamber of the catheter. After the fluid vaporizes and expands in the distal chamber and cools the walls, it is returned through the body of the catheter to a coolant collection system, preferably in the form of a recirculation loop.
For such a cryocatheter, coolant is released at high pressure in a relatively small chamber at the tip of the catheter and recirculates back via a return conduit from the tip region. For cardiac ablation, the injection is controlled from a low rate of delivery for cold mapping or treatment site confirmation, to a higher rate of delivery used for tissue ablation at the mapped or confirmed sites. Thermal transfer may vary as ice accumulates on the tip. For other applications such as thermal angioplasty, proper treatment may require precise control of the cooling in other temperature ranges. The wide range of required energy transfer rates as well as differences in size, shape or construction of different catheters increases the difficulty of achieving uniform or repeatable catheter cooling rates. This has resulted in instruments that operate in restricted temperature ranges and with wide variations in their cooling characteristics.
RF ablation catheters for ablating tissue and cardiac treatment by the localized application of RF energy are of similar size, and typically employ a catheter tip construction in which a monopolar or a bipolar (split) electrode tip applies an AC electrical signal to tissue in contact with the electrode. In this technology, cooling fluid may also be applied to prevent excessive heating of the electrode itself, or to chill tissue and allow cold-mapping during a treatment regimen. Other special constructions such as the use of an electrically conductive saline irrigant, may be used to extend the size of the lesion, and cardiac signal sensing electrodes may also be spaced along the length of the tip, allowing a single instrument to detect and map cardiac signals during treatment. However, RF catheters typically operate quite locally. Resistive tissue heating falls off with the fourth power of distance, and while electrode cooling may somewhat change their heating characteristics, their limited range of operation often necessitates lengthy treatment procedures involving many iterations of cold mapping, ablative lesion forming, and re-mapping or checking steps. The necessary number of steps may require over an hour to perform.
Accordingly, there remains a need for a catheter construction that achieves an extended range of thermal transfer.
There is also a need for a cryocatheter construction that ablates tissue more effectively, or to a greater depth.
There is also a need for a cryocatheter construction that is controllable to provide uniform and repeatable thermal treatment over a wider range of thermal energy transfer conditions.